Age-dependent synergistic control of sit-to-stand motion

Sit-to-Stand (STS) is a demanding postural task whose performance becomes challenging with aging. While muscle synergy analyses have helped describe its modular control, most evidence is based on unilateral EMG, which can characterize within-limb structure but cannot test whether the two sides share stable motor modules or reveal subtle inter-limb compensations in an apparently symmetric task. This study investigated the effects of aging on the neuromuscular organization of STS by examining bilateral muscle synergies and their relation to the whole-body center-of-mass (CoM) kinematics.

Seventeen young and sixteen older healthy adults performed a 30-s STS test while kinematic data and bilateral muscle activity from seven trunk and lower-limb muscles were collected. Synergies were extracted using non-negative matrix factorization and linked to CoM behavior through a mapping approach that associates synergy recruitment with regions of the CoM kinematic state space.

Four synergies were sufficient to reconstruct the activation patterns, with highly consistent structure between limbs and no age-related differences in synergy number or spatial structure. Older adults, however, showed broader activation duration (p = 0.02) and earlier recruitment timing (p = 0.009) of the synergy associated with trunk flexion and forward momentum generation. Furthermore, this anticipatory shift was consistent with the elderly's CoM trajectory entering the corresponding high-probability region earlier along the anteroposterior axis in the CoM-synergy mapping.

These findings suggest that while the fundamental bilateral modular architecture of STS is preserved with aging, its temporal precision and coupling with whole-body mechanics become less distinct, reflecting adaptive strategies to maintain stability during postural transitions.